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Dose humid warm air rise faster then warm dry air?


Buoyancy is dependent on a difference in density between an air parcel and the surrounding air.

Assuming that you have two air parcels of the same volume, the warmer parcel will be less dense and more buoyant. This also implies that the warmer air parcel exerts more pressure outwardly than does a cooler air parcel (this effect causes hot air balloons to expand as they warm).

Increasing the amount of water vapor in an air parcel (assuming that no condensation of water is taking place) makes it less dense, as water molecules (which have a lower mass than air) displace air molecules.

Therefore, humid air would rise faster than dry air, assuming that they both started with the same volume and temperature.

As both parcels rise, they will both cool at the same rate with height (the dry adiabatic lapse rate); both will continue to rise until they become cooler than the surrounding air.

However, if the humid air cools to the point of becoming saturated (where condensation of water droplets begins), at that point it would cool more slowly with height (since heat is released as water is condensed out from the air). This reduction in cooling with height means that it can have a higher temperature and an even lower density than the dry parcel. The humid parcel can then rise even more rapidly than it did before, while the dry parcel is still rising at it's original rate.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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